Move duplicate Tick() code from Plane and Helicopter to Aircraft.

OpenRA.Mods.Common/Traits/Air/Aircraft.cs

@@ -77,6 +77,29 @@ namespace OpenRA.Mods.Common.Traits
 			Facing = init.Contains<FacingInit>() ? init.Get<FacingInit, int>() : info.InitialFacing;
 		}
 
+        bool firstTick = true;
+        public virtual void Tick(Actor self)
+        {
+            if (firstTick)
+            {
+                firstTick = false;
+
+                // TODO: Aircraft husks don't properly unreserve.
+                if (self.HasTrait<FallsToEarth>())
+                    return;
+
+                ReserveSpawnBuilding();
+
+                var host = GetActorBelow();
+                if (host == null)
+                    return;
+
+                self.QueueActivity(new TakeOff(self));
+            }
+
+            Repulse();
+        }
+
 		public void Repulse()
 		{
 			var repulsionForce = GetRepulsionForce();